1. Field of the Invention
This invention relates to digital video tape recorder (VTR) apparatus.
2. Description of the Prior Art
Sample shuffling has become a well-known technique for use in digital VTR apparatus as a method of reducing the adverse effects of tape drop-out on a reproduced television picture. In sample shuffling, the video samples corresponding to say a field of the television signal are not recorded in the sequence corresponding to the sample order in each horizontal scan line and the order of the horizontal scan lines, but are shuffled in some predetermined way prior to recording on the magnetic tape and are deshuffled after reproduction from the magnetic tape. This ensures that a group of error samples resulting, for example, from a drop-out, are distributed over some period, such as a field, of the reproduced television signal, and this substantially increases the possibility of being able to conceal the error samples using some interpolation process.
Although sample shuffling is effective in the case of drop-out, there are still two particular circumstances where one or more stationary noise bars or a stationary noise pattern may occur in a reproduced television picture. The first is where there is a longitudinal scratch on the magnetic tape, and the second is where the digital VTR apparatus is used in shuttle mode, that is, to reproduce at a forward or reverse speed in excess of the normal reproduction speed.
These problems will now be described in more detail with reference to FIGS. 1 to 6 of the accompanying drawings.
FIG. 1 shows diagrammatically skew tracks 1 on a magnetic tape 2. An integral number, which may be one, of the tracks 1 correspond to each field of the recorded television signal. In the present description it will be assumed that three tracks 1 correspond to each field. Moreover, it is assumed that the tape 2 has suffered a longitudinal scratch 3, such as may result, for example, from a burr on a tape guide in the tape transport mechanism. Finally, it will be assumed that the tracks 1 are scanned by a reproducing head 4 which scans each track 1 as indicated by the arrows 5.
FIG. 2 shows diagrammatically the result of the scratch 3 (FIG. 1) on a reproduced television picture 6, it being assumed that sample shuffling has not been used. Because three of the tracks 1 correspond to one field, and because the scratch 3 has corrupted or destroyed the recorded sample signals in corresponding regions of each track 1, the reproduced television picture 6 will have three noise bars 7. Any concealment process used will be ineffective in this situation, because concealment is necessarily effected by interpolation and this relies on the availability of uncorrupted samples adjacent to any error sample, adjacent in this case meaning adjacent in the same horizontal scan line or adjacent in an adjacent horizontal scan line or in a corresponding position in an adjacent field in the sequence of fields. Because the scratch 3 affects corresponding regions of each track 1, such adjacent uncorrupted samples will not be available in the case of many of the error samples resulting from the scratch 3.
FIG. 3 again shows diagrammatically the result of the scratch 3 (FIG. 1) on the reproduced television picture 6, but in this case it is assumed that sample shuffling has been used. As a consequence it is likely that some of the error samples will be concealed, but there will still remain a significant number that it is not possible to conceal, and these will result in a stationary noise pattern 8 on the reproduced television picture 6.
Similar problems arise in the shuttle mode. Thus, as indicated diagrammatically in FIG. 4, if the recorded signal is being reproduced at high speed, the reproducing head 4 will trace a path 9 which crosses the tracks 1. In consequence, the envelope of the radio frequency reproduced signal will be generally as indicated in the graph of FIG. 5, with the amplitude of the reproduced signal rising from a minimum 10 to a maximum 11 as the reproducing head 4 moves from the edge to the centre of a track 1, and vice versa.
As indicated diagrammatically in FIG. 6, the effect of this on the reproduced television picture 6 is that there are 3n noise bars 12, where n is the multiple by which the reproduction speed deviates from the normal reproduction speed. Concealment is wholly ineffective in such a case, simply because the density of error samples is too large for any effective interpolation.